dt-bindings: iio: light: add UVIS25 device bindings

* ST UVIS25 uv sensor

Required properties:

- compatible: should be "st,uvis25"

- reg: i2c address of the sensor / spi cs line

Optional properties:

- interrupt-parent: should be the phandle for the interrupt controller

- interrupts: interrupt mapping for IRQ. It should be configured with

  flags IRQ_TYPE_LEVEL_HIGH, IRQ_TYPE_EDGE_RISING, IRQ_TYPE_LEVEL_LOW or

  IRQ_TYPE_EDGE_FALLING.

  Refer to interrupt-controller/interrupts.txt for generic interrupt

  client node bindings.

Example:

uvis25@47 {

	compatible = "st,uvis25";

	reg = <0x47>;

	interrupt-parent = <&gpio0>;

	interrupts = <0 IRQ_TYPE_EDGE_RISING>;

};